关于xGW(PDSN)，配置了主备AAA，由于主用AAA故障，业务切换到备用AAA，回切时需要将aaa-profile工作模式配置为（）。 On the xGW(PDSN), both the master AAA and the slave AAA are configured. After services are switched over to the slave AAA when the master AAA becomes faulty, we need to set the work mode of the aaa-profile to ( ) during switchback.
选项：

A:load-balance
B:primary-prior
C:primary-standby
D:round-robin